The present invention deals with a new protein activator of CREM in Testis named ACT, the cDNA encoding ACT and the use of said protein and cDNA for screening modulating compounds which are able to interact with ACT or its site of interaction on CREM to block or stimulate the CREM transcriptional activity in Testis. Such compounds may lead to control of male fertility.
Transcriptional activation by CREB and CREM requires serine phosphorylation within the activation domain (Ser133 in CREB; Ser117 in CREM) and consequent interaction with the co-activator CBP1, 2. The activator CREM is expressed at very high levels in male germ cells and is required for post-meiotic gene expression2-4. Using a two-hybrid screen, we have isolated a testis-derived cDNA encoding ACT (Activator of CREM in Testis), a novel LIM-only protein that specifically associates with CREM. ACT is expressed coordinately with CREM in a highly tissue- and developmentally-regulated manner. ACT powerfully stimulates CREM transcriptional activity in yeast and mammalian cells and it contains an intrinsic activation function. Strikingly, ACT bypasses the classical requirements for activation, namely Ser117-phosphorylation and interaction with CBP. Thus, ACT identifies a novel pathway of transcriptional activation by CREM and CREB. ACT may define a novel class of tissue-specific co-activators whose function could be specific of distinct cellular differentiation programs.
CREB and CREM are factors directly coupled to signaling pathways. Phosphorylation within the P-box, and consequent CBP interaction, turns them into powerful activators1, 2. The activation domain (AD) comprises also two glutamine-rich regions Q1 and Q2 interaction to the TBP-associated hTAF130 seems involved in activation5. While CREB is ubiquitous, CREM displays a remarkable tissue distribution and plays important roles in the nuclear response to physiological stimuli2. In particular, the activator CREM is hundred-fold more abundant in male germ cells than in any other tissue4. CREM controls the transcription of various post-meiotic genes2,3 and its targeted inactivation in the mouse results in the complete block of spermatogenesis at early spermatids stage6, 7. Unexpected recent results show that the activator CREM is not phosphorylated at Ser117 in germ cells (in preparation). Thus, while the physiological role of CREM in germ cells is well established, the molecular mechanism by which it exerts its function remains elusive.
We wished to investigate how activation by CREM could occur in the absence of phosphorylation. A yeast two-hybrid was used to screen a murine testis cDNA expression library with the CREM AD (FIG. 1a). The CREM AD fused to the GAL4 DBD (DNA-binding domain) is completely inactive (FIGS. 1b; 1c) for the lack of CBP and hTAF130 yeast homologs. One clone interacted with high affinity when tested for nutritional selection and xcex2-galactosidase activity (FIGS. 1b; 1c). This encodes a novel 284aa protein (SEQ ID NO:1) FIG. 1d) ACT (activator of CREM in testis), containing four complete and one N-terminal half LIM motif (FIG. 1d). LIM domains are constituted by double zinc-finger structures, are present in various proteins and thought to mediate protein-protein interactions8, 9.
ACT displays a remarkable tissue-specific expression (FIG. 2a). We have raised anti-ACT specific antibody which revealed a protein of the expected size (33 kDa). ACT protein colocalizes in purified spermatids with CREM10 (FIG. 2b). In situ analysis shows regulated expression during germ cell differentiation (FIG. 2c), coordinated with CREM10. Immunohistochemistry demonstrates that ACT is nuclear and expressed in round and elongated spermatids (FIG. 2d). Thus, CREM and ACT proteins are coexpressed in vivo.
ACT and CREM proteins efficiently associate (FIG. 3). Purified full-length ACT fused to glutathione-S-transferase (GST) was tested for binding to different CREM isoforms. The presence of the sole P-box is sufficient for binding ACT (FIG. 3a). Consistent with the yeast two-hybrid assay, ACT does not bind to the repressor ICER11, indicating that the DBD is dispensable for interaction. To map the region involved in CREM association, we generated ACT truncations in each LIM motif. Mutants lacking the fourth LIM domain (GST-ACT 1-221) or the N-terminal half LIM domain (GST-ACT 38-284) interact with CREM, while deletion of the two C-terminal LIM domains (GST-ACT 1-162) impairs association (FIG. 3b). Thus, the third LIM domain appears critical for CREM-ACT interaction. CREM-ACT association is also revealed by co-immunoprecipitation after expression in mammalian cells (FIG. 3c). Extracts from cells transfected with CREM and Myc-tagged ACT expression vectors, individually or in combination, were immunoprecipitated using anti-CREM antibodies. Western analysis (FIG. 3c) revealed that ACT associates with CREM in immunoprecipitated complexes. No ACT protein was co-immunoprecipitated by anti-CREM antibodies when ACT and CREM were expressed separately. Conversely, CREM-ACT association was detected when the Myc-tagged ACT protein was immunoprecipitated from co-transfected cells, using an anti-Myc antibody (FIG. 3c). Finally, the CREM-ACT association is also revealed by co-immunoprecipitation of native proteins from a testis extract (FIG. 3d).
Sequence analysis of the ACT cDNA from the two-hybrid screening held a surprise: the coding sequence was not in frame with the GAL4 AD within the yeast expression vector. Thus ACT could be translated from its own AUG and therefore have intrinsic coactivator properties in yeast. To test this, the GAL4-AD was deleted generating ACT(delGal4-AD) (FIG. 4a). Strikingly, ACT alone powerfully activates CREM-dependent transcription. Thus ACT bypasses the requirement of CBP for activation, as CBP is lacking in yeast. A fusion of ACT in frame with GAL4 AD (ACT/Gal4-AD) elicits only a two-fold higher activity than ACT, indicating that ACT, per se, has an efficient activation potential. To assess whether ACT has an intrinsic AD we fused it to the GAL4 DBD (ACT/Gal4-DBD). This fusion protein efficiently induces activation (FIG. 4a). Thus, ACT bears an autonomous AD as its recruitment to DNA is sufficient to elicit activation.
The CREM gene generates both activators and repressors2,12. Is ACT able to modify the repressors function? In a two-hybrid assay ACT-mediated co-activation takes place only in the presence of CREM activator isoforms, as the P-box in combination with at least one glutamine-rich domain is required (CREMxcfx841 and CREMxcfx842, FIG. 4b).
ACT is a powerful activator in mammalian cells. Using a GAL4-based reporter13 CREM-mediated activation was enhanced by ACT in a dose-dependent manner. Strikingly, ACT functions in absence of co-expressed PKA catalytic subunit (FIG. 4c). Analogous results were obtained with full-length CREM and CREB on a CRE-driven somatostatin reporter and the testis-specific angiotensin converting enzyme (ACE) and calspermin promoters14,15 (FIG. 4d,e; not shown). ACT displays inherent activating function also in mammalian cells, as ACT fused to the GAL4 DBD strongly stimulates transcription from the G4CAT reporter (FIG. 4f).
We wished to test ACT function on CREB, as it is known that GAL4 DBD fusion proteins containing the glutamine-rich activation domains are inactive in yeast16. We show that ACT significantly activates CREB-mediated transcription both in yeast and in mammalian cells (FIG. 4g; and not shown).
Phosphorylation at Ser133 in CREB and Ser117 in CREM is critical for activation and CBP interaction17-21. As ACT bypasses the CBP requirement (FIG. 4), we wondered whether a Ser greater than Ala substitution may prevent ACT function. The Ser greater than Ala117 mutation did not affect the CREM-ACT physical association tested by co-immunoprecipitation after transfection in mammalian cells (FIG. 5a). Strikingly, the Ser greater than Ala117 mutation did not decrease the stimulating ACT function, both in yeast and mammalian cells (FIGS. 5b, c). Analogous results were obtained with a CREB Ser greater than Ala133 mutation (not shown). Thus, ACT bypasses the phosphorylation requirement for activation.
Remarkable modulations in gene expression underlie the dramatic changes occuring during germ cells differentiation3. Unlike CBP, p300 and hTAF130, which are ubiquitous, ACT shows a stringent tissue-specific and temporal expression coordinated with CREM. Interactions between LIM-only proteins and transcription factors have been described in some differentiating cell systems8, 22-25. Here we provide the first evidence that LIM domains can bear an intrinsic activation potential. Moreover, ACT bypasses the classical requirement of activation by CREB and CREM. Indeed, a striking difference with CBP is that ACT association and function do not require phosphorylation at Ser117. The molecular mechanism by which ACT functions is intriguing. It is likely that ACT needs to interact with basal transcription factors or coactivators that must be conserved between yeast and mammalian cells. The absence of a CBP homolog in yeast demonstrates that ACT functions in a CBP-independent manner. Finally, the possible existence of other tissue-specific ACT-like molecules may suggest that ACT represents the first example of a novel class of co-activators whose function is specific of distinct steps of cellular differentiation programs.
So, the present invention deals with activator of CREM in Testis (ACT) having the formula: (SEQ ID NO:1)
1 MTSSQFDCQY CTSSLIGKKY VLKDDNLYCI SCYDRIFSNY CEQCKEPIES
51 DSKDLCYKNR HWHEGCFRCN KCHHSLVEKP FVAKDDRLLC TDCYSNECSS
101 KCFHCKRTIM PGSRKMEFKG NYWHETCFVC EHCRQPIGTK PLISKESGNY
151 CVPCFEKEFA HYCNFCKKVI TSGGITFRDQ IWHKECFLCS GCRKELYEEA
201 FMSKDDFPFC LDCYNHLYAK KCAACTKPIT GLRGAKFICF QDRQWHSECF
251 NCGKCSVSLV GEGFLTHNME ILCRKCGSGA DTDA
or fragments thereof which are able to interact with CREM.
Said fragments or full-length ACT may be inserted within a protein, especially the active epitope of ACT may be inserted within protein to be used as marker of any other use. If it is used for imaging CREM the marker may be fluorescent or radioactive.
The present invention also deals with antibodies specific for the above protein which may be useful to interact with ACT or to prepare a diagnostic kit for identification of ACT in natural samples.
The invention also deals with DNA sequence coding the ACT protein, particularly the natural ACT DNA of FIG. 6.
Invention also deals with vectors expressing the above protein and cells comprising said vector and expressing the corresponding protein.
The expression vectors of the invention may require polynucleotide sequences which are necessary to effect the expression of coding sequences to which they are ligated to (i.e. the DNA sequence of the invention encoding for ACT or a fragment thereof). The nature of such control sequences differs depending upon the host organism; in eukaryotes, generally, such control sequences include promoter, enhancer, polyadenylation sequences and transcription termination sequence. The elements required for the expression are intended to include, at the minimum, all components whose presence is necessary for expression, and may also include additional components whose presence is advantageous. The vectors containing the nucleic acid sequence of interest can be transferred into the host cell by well-known methods, which vary depending on the type of the cellular host. For exemple, calcium phosphate, electroporation and lipofection may be used. These expression vectors are typically replicable in the host cells either as episomes or as integral part of the host chromosomal DNA. Host cells of the invention are chosen among bacterial cells, yeast cells, animal cells such as mammalian cells. In a preferred embodiment, the host cells are yeast cells.
An embodiment of the invention provides a method for selecting a compound or a compound likely to affect the interaction between the ACT protein or a fragment thereof and the CREM protein. This method of screening may be done in vitro with the corresponding proteins or in vivo, by using for example the two hybrid process, as described in the examples. In a preferred embodiment, this method is based on the two-hybrid assay . In this method, the compound of the invention is likely to interact with the protein ACT or a fragment thereof and/or to interact with the protein CREM or a fragment thereof, in order to form a complex likely to alter the transcriptional activity of genes which expression is regulated by ACT and/or CREM polypeptides. Said compound is likely to increase, decrease, modulate or void said transcriptional activity. In a preferred embodiment, the selected compound of the invention causes the transcriptional activity to decrease.
The method of the invention comprises the following steps:
a) providing a recombinant host cell containing a reporter gene, wherein the reporter gene expresses a detectable polypeptide when the reporter gene is activated by an amino acid sequence including a transcriptional activation domain, e.g., GAL4;
b) treating the host cell with a compound for which the capacity to alter the interaction between ACT protein or a fragment thereof and a CREM polypeptide is tested.
c) providing a chimeric ACT gene that is capable of being expressed in host cell, the chimeric ACT gene comprising a DNA sequence that encodes a ACT hybrid polypeptide, said ACT hybrid polypeptide comprising:
(i) the transcriptional activation domain; and
(ii) the ACT protein or a fragment thereof which is able to interact with CREM or a fragment thereof.
d) providing a CREM chimeric gene that is capable of being expressed in the host cell, the CREM chimeric gene comprising a DNA sequence that encodes a CREM hybrid polypeptide, the CREM hybrid polypeptide comprising:
(i) a DNA-binding domain, such as GAL4 that recognizes a binding site on the reporter gene in the host cell, and
(ii) a CREM protein or a fragment thereof which are able to interact with the ACT proteins or fragments thereof.
Wherein interaction between the ACT hybrid polypeptide and the CREM hybrid polypeptide in the host cell under physiological conditions causes the transcriptional activation domain to activate transcription of the reporter gene.
e) introducing said chimeric ACT gene, said CREM chimeric gene and said compound into the host cell;
f) subjecting the host cells to conditions under which the ACT hybrid polypeptide and the CREM hybrid polypeptide are expressed in sufficient quantity for the reporter gene to be activated.
g) selecting the compound introduced into the host cell, that allows the reporter gene to be expressed to a degree lesser than its expression level in the absence of the said compound.
As used herein, the terms xe2x80x9cinteracting polypeptide sequencexe2x80x9d refer to a portion of a ACT hybrid polypeptide which can form a specific binding interaction with a portion of a CREM hybrid polypeptide under suitable physiological conditions.
As used herein the term xe2x80x9cphysiological conditionsxe2x80x9d refers to temperature, pH, ionic strength, viscosity, and like biochemical parameters which are compatible with a viable organism, and/or which typically exist intracellularly in a viable cultured yeast cell or mamalian cell. For example, the intracellular conditions in a yeast cell grown under typical laboratory culture conditions are physiological conditions. Suitable in vitro reaction conditions for in vitro transcription cocktails are generally physiological conditions. In general, in vitro physiological conditions comprise 50-2000 mM NaCl or KCl, pH 6.5-8.5, 20-45xc2x0 C. and 0.01-10 mM divalent cation (e.g., Mg++, Ca++); preferably about 150 mM NaCl or KCl, pH 7.2-7.6, 5 mM divalent cation, and often include 0.01-1.0 percent nonspecific protein (e.g., BSA). Particular aqueous conditions may be selected by the practitioner according to conventional methods. For general guidance, the following buffered aqueous conditions may be applicable: 10-250 mM NaCl, 5-50 mM Tris HCl, pH 5-8 with optional addition of divalent cation(s) and/or metal chelators and/or nonionic detergents and/or membrane fractions.
According to a preferred embodiment, the reporter protein encoded by the reporter gene is expressed in control conditions (e.g., physiological conditions in the absence of agent) wherein the two hybrid proteins bind to each other and form a transcriptional activator that triggers or increases transcription of the reporter gene. Host organisms harboring such a two-hybrid system are cultured in the presence of said compound, and expression of the host organism reporter gene is determined and standardized to a parallel blank culture which lacks said compound. Compounds which produce a significant decrease in expression of the reporter gene in a host organism after a suitable time period (e.g., usually at least 1 hour, often at least 3 hours, preferably about 6 hours, occasionally overnight or longer) are thereby identified as inhibitors for blocking the intermolecular association between the ACT and CREM polypeptides.
For example and without limitation, suitable detectable genes are those which (1) confer a selectable phenotype to cells in which the detectable gene is efficiently expressed, and/or (2) encode a gene product (e.g., enzyme) which is conveniently detected such as by in situ assay or the like. Suitable genes which confer a selectable phenotype are exemplified by, but not limited to, genes which complement auxotrophic mutations in a host organism (e.g., yeast HIS3), genes which encode drug resistance (e.g., neoR), genes which induce cell proliferation, and other genes whose expression confers a selective growth advantage. Suitable genes which encode a gene product which is conveniently detected in situ are exemplified by, but not limited to, xcex2-galactosidase (e.g. E. coli lacz), luciferase, alkaline, phosphatase, horseradish peroxidase, and the like.
In a preferred embodiment of method of the invention, yeast cells are the host organism, the detectable gene encodes xcex2-galactosidase and/or a protein that complements an auxotrophic mutant yeast host cell, and the ACT and CREM hybrid polypeptides each comprise a binding domain derived from a signal transduction protein.
The compound tested and selected by the method of the invention include, but is not limited to, a chemical compound, a mixture of chemical compounds, a biological macromolecule such as a polypeptide, oligonucleotide, small biological molecule, (typical MW less than 5,000, preferably  less than 1,000), and extract made from biological material such as bacteria, plants, fungi or animal (particularly mammalian) cells or tissues.
In a preferred embodiment, compounds are evaluated for potential activity as specific protein interaction inhibitors (that is to say an agent which selectively inhibits a binding interaction between ACT and CREM polypeptides but which does not substantially interfere with cell viability) by inclusion in screening assays described herein above. Compounds selected by the method of the invention, and which act as a ACT-CREM interaction inhibitors may have therapeutic potential as drugs for human use and/or may serve as commercial reagents for laboratory research or bioprocess control. Candidate drugs selected by the method of the invention are then tested further for activity in assays which are routinely used to predict suitability for use as human and veterinary drugs, including in vivo administration to non-human animals and often including administration to human in approved clinical trials.
In a preferred embodiment, ACT-CREM interaction inhibitors selected by the method of the invention are human or animal drugs useful to control male fertility. The drug of the invention can be administered orally, rectally, parenterally, or by local route, particularly for a man, for exemple by percutaneous route, or by injection, in particular subcutaneous, in the veterinary field. They can be in the form of tablets, dragees, capsules, granules, suppositories, injectable preparations, pressaries, creams, gels, microspheres, implants and patches prepared by the usual methods. Examples of suitable excipients are talc, gum arabic, lactose, starch, magnesium, stearate, cocoa butter, aqueous or non aqueous vehicle, fatty substances of animal or vegetable origin, paraffin derivatives, glycols, or emulsifying agents and preservatives.
In a preferred embodiment, the drug of the invention is biodegradable such that its potential activity as specific inhibitor is effective for a definite time in the organism. Accordingly, suitable excipients are used to modulate in vivo half-live of the drug of the invention.
It is also possible to use ACT, ACT-like compounds (for example proteins) or compound selected by the above-mentionned method, with a potential activity as ACT-CREM interaction activators to stimulate the CREM transcriptional activity in Testis.
Finally, the present invention deals with pharmaceutical composition comprising at least one of the above proteins or compounds or vectors expressing ACT or ACT-like compounds.
Another approach can be envisionned to control male fertility. This method is based on an immunological response to ACT using for exemple the antibodies of the invention.
The following examples will help to understand the above invention.